How far can a government change the age to be an adult?

In the US, the government defined adulthood (for purposes of drinking alcohol) as 21 several years ago, but kept it at 18 for everything else.

In general, the government can do anything it damn well pleases, subject only to the limits of the constitution and the electorate.

Remember that there are societal issues with keeping kids in school – some families need the kids to work to make an income to support the family.

The age to drive is a standard 16 years.
Marriage ages vary with different states and their requirements.
Draft age in times of war was Eighteen, if out of school.
Drinking was 18, then raised due to statistics and pressure, to 21.
Most drinking ages went down to 18 again during the Vietnam War- ‘Old enough to fight/die, old enough to drink.’
Since then, most all have raised it back to 21.
Ages vary on various smaller vehicle operation, etc. State’s rights variance.
Age to buy a gun to shoot- two, if you find the right seller….